2. 95-702 OCT 1 Master of Information System Management Organizational Communications and Distributed Object Technologies Lecture 5: Distributed Objects & Event Notification

3. 95-702 OCT 2 Master of Information System Management Middleware layers Applications Middleware layers Request reply protocol External data representation Operating System RMI, RPC and events

4. 95-702 OCT 3 Master of Information System Management Traditional Interfaces Interfaces promote modularity. Recall the use of c header files. One module may access another module without concern over implementation details. Method signatures are specified. The compiler need only consider signatures when compiling the caller.

5. 95-702 OCT 4 Master of Information System Management Interface Definition Language Definition: An interface definition language (IDL) provides a notation for defining interfaces in which each of the parameters of a method may be described as for input or output in addition to having its type specified. These may be used to allow objects written in different languages to invoke one another.

6. 95-702 OCT 5 Master of Information System Management Interface Definition Language A language independent IDL can be used bridge the gap between programming languages. Examples include:  Corba IDL (Object-oriented syntax)  OSF’s Distributed Computing Environment DCE (C like syntax)  DCOM IDL based on OSF’s DCE and used by Microsoft’s DCOM  Sun XDR (An IDL for RPC)  Web Services WSDL In the case of Web Services, how is WSDL different from XSDL?

7. 95-702 OCT 6 Master of Information System Management CORBA IDL example // In file Person.idl struct Person { string name; string place; long year; } ; interface PersonList { readonly attribute string listname; void addPerson(in Person p) ; void getPerson(in string name, out Person p); long number(); }; How does this compare with WSDL?

8. 95-702 OCT 7 Master of Information System Management File interface in Sun XDR (Originally External Data Representation but now an IDL) for RPC const MAX = 1000; typedef int FileIdentifier; typedef int FilePointer; typedef int Length; struct Data { int length; char buffer[MAX]; }; struct writeargs { FileIdentifier f; FilePointer position; Data data; }; struct readargs { FileIdentifier f; FilePointer position; Length length; }; program FILEREADWRITE { version VERSION { void WRITE(writeargs)=1; // procedure Data READ(readargs)=2; // numbers }=2; // version number } = 9999; // program number // numbers passed in request message // rpcgen is the interface compiler

9. 95-702 OCT 8 Master of Information System Management Traditional Object Model Each object is a set of data and a set of methods. Object references are assigned to variables. Interfaces define an object’s methods. Actions are initiated by invoking methods. Exceptions may be thrown for unexpected or illegal conditions. Garbage collection may be handled by the developer (C++) or by the runtime (.NET and Java)

10. 95-702 OCT 9 Master of Information System Management Distributed Object Model Having client and server objects in different processes enforces encapsulation. You must call a method to change its state. Methods may be synchronized to protect against conflicting access by multiple clients. Objects are accessed remotely (by message passing) or objects are copied to the local machine (if the object’s class is available locally) and used locally. Remote object references are analogous to local ones in that: 1. The invoker uses the remote object reference to identify the object and 2. The remote object reference may be passed as an argument to or return value from a local or remote method.

11. 95-702 OCT 10 Master of Information System Management Remote and Local Method Invocations invocation remote invocation remote local invocation A B C D E F

12. 95-702 OCT 11 Master of Information System Management A Remote Object and its Remote Interface interface remote m1 m2 m3 m4 m5 m6 Data implementation remoteobject { of methods

13. 95-702 OCT 12 Master of Information System Management RMI Design Issues RMI Invocation Semantics Local calls have Exactly Once semantics. Remote calls have Maybe, At Least Once or at Most Once semantics. Different semantics are due to the fault tolerance measures applied during the request reply protocol. Level of Transparency Remote calls should have a syntax that is close to local calls. But it should probably be clear to the programmer that a remote call is being made.

14. 95-702 OCT 13 Master of Information System Management Invocation Semantics Fault tolerance measures Invocation semantics Retransmit request message Duplicate filtering Re-execute procedure or retransmit reply No Yes Not applicable No Yes Not applicable Re-execute procedure Retransmit reply (history)At-most-once At-least-once Maybe Duplicate filtering means removing duplicate request at the server.

15. 95-702 OCT 14 Master of Information System Management Invocation Semantics Maybe semantics is useful only for applications in which occasional failed invocations are acceptable. At-Least-Once semantics is appropriate for idempotent operations. At-Most-Once semantics is the norm.

16. 95-702 OCT 15 Master of Information System Management Generic RMI Modules object A object B skeleton Request proxy for B Reply Communication Remote Remote reference Communication module reference module module for B’s class & dispatcher remote client server

17. 95-702 OCT 16 Master of Information System Management The Remote Reference Module object A object B skeleton Request proxy for B Reply Communication Remote Remote reference Communication module reference module module for B’s class & dispatcher remote client server The remote reference module holds a table that records the correspondence between local object references in that process and remote object references (which are system wide).

18. 95-702 OCT 17 Master of Information System Management The Communication Module object A object B skeleton Request proxy for B Reply Communication Remote Remote reference Communication module reference module module for B’s class & dispatcher remote client server Coordinate to provide a specified invocation semantics. The communication module selects the dispatcher for the class of the object to be invoked, passing on the remote object’s local reference.

19. 95-702 OCT 18 Master of Information System Management Proxies object A object B skeleton Request proxy for B Reply Communication Remote Remote reference Communication module reference module module for B’s class & dispatcher remote client server The proxy makes the RMI transparent to the caller. It marshals and unmarshals parameters. There is one proxy for each remote object. Proxies hold the remote object reference.

20. 95-702 OCT 19 Master of Information System Management Dispatchers and Skeletons (1) object A object B skeleton Request proxy for B Reply Communication Remote Remote reference Communication module reference module module for B’s class & dispatcher remote client server The server has one dispatcher and skeleton for each class representing a remote object. A request message with a methodID is passed from the communication module. The dispatcher calls the method in the skeleton passing the request message. The skeleton implements the remote object’s interface in much the same way that a proxy does. The remote reference module may be asked for the local location associated with the remote reference.

21. 95-702 OCT 20 Master of Information System Management Dispatchers and Skeletons (2) object A object B skeleton Request proxy for B Reply Communication Remote Remote reference Communication module reference module module for B’s class & dispatcher remote client server The communication module selects the dispatcher based upon the remote object reference. The dispatcher selects the method to call in the skeleton. The skeleton unmarshalls parameters and calls the method in the remote object.

22. 95-702 OCT 21 Master of Information System Management Generic RMI Summary object A object B skeleton Request proxy for B Reply Communication Remote Remote reference Communication module reference module module for B’s class & dispatcher remote client server RMI software - between application level objects and communication and remote reference modules Proxy - makes RMI transparent to client. Class implements remote interface. Marshals requests and unmarshals results. Forwards request. Dispatcher - gets request from communication module and invokes method in skeleton (using methodID in message). Skeleton - implements methods in remote interface. Unmarshals requests and marshals results. Invokes method in remote object. carries out Request- reply protocol translates between local and remote object references and creates remote object references. Uses remote object table

23. 95-702 OCT 22 Master of Information System Management Binders object A object B skeleton Request proxy for B Reply Communication Remote Remote reference Communication module reference module module for B’s class & dispatcher remote client server Java uses the rmiregistry CORBA uses the CORBA Naming Service Binders allow an object to be named and registered.

24. 95-702 OCT 23 Master of Information System Management Local Events and Notifications Examples of the local event model: (1) A keystroke causes an interrupt handler to execute, storing a key character in the keyboard buffer. (2) A mouse click causes an interrupt handler to call a registered listener to handle the mouse event.

25. 95-702 OCT 24 Master of Information System Management Distributed Event Based System Suppose a whiteboard server is willing to make calls to all registered clients when the drawing is changed by any one client. Clients may subscribe to this service (register interest). The whiteboard server publishes the events that it will make available to clients. This is the publish-subscribe paradigm

26. 95-702 OCT 25 Master of Information System Management Two Characteristics of Distributed Event Based Systems (1) Heterogeneous -- event generators publish the types of events they offer -- other objects subscribe and provide callable methods -- components that were not designed to work together may interoperate

27. 95-702 OCT 26 Master of Information System Management Two Characteristics of Distributed Event Based Systems (2) Asynchronous -- Publishers and subscribers are decoupled -- notifications of events are sent asynchronously to all subscribers

28. 95-702 OCT 27 Master of Information System Management Dealing room system

29. 95-702 OCT 28 Master of Information System Management Architecture for distributed event notification subscriberobserverobject of interest Event service object of interest observer subscriber 3. 1. 2. notification